1. Field of the Invention
The invention relates to a system for utility vehicles, in particular agricultural tractors, for supplying primary and auxiliary pressure medium consumers with different system pressures, which are built up as a function of the load pressure of the pressure medium consumers and which exceed the load pressure by a pre-determined control pressure differential. In order to produce a first control pressure differential for operating a primary pressure medium consumer, its load pressure is used directly for building up the system pressure and in order to produce a second, higher control pressure differential for operating an auxiliary pressure medium consumer, its load pressure is converted in an amplifying device to an increased pressure, which is used for building up a higher system pressure.
2. Description of Related Art
European Patent EP 10 70 852 A2 describes a hydraulic system, which can be equipped with a load sensing pump or a fixed displacement pump and whose system pressure and flow rate are adapted to the respective need by a pressure and flow controller. If a fixed displacement pump is used the pressure and flow controller is adjusted as a function of the maximum load pressure of the primary pressure medium consumers so that the system pressure is always higher by a pressure differential of approximately 10 bar than the maximum load pressure. For operating auxiliary pressure medium consumers due to the greater pressure losses in the supply lines the pressure and flow controller is adjusted in such a way that the system pressure is always higher by a pressure differential of approximately 20-23 bar than the maximum load pressure. To create the higher control pressure differential for auxiliary pressure medium consumers an amplifying device is provided, which produces an actuating pressure exceeding the maximum load pressure, to which the pressure and flow controller is subjected. To this end for operating both the primary and auxiliary pressure medium consumers the actuating pressure between two throttling restrictions of the amplifying device is reduced. To create different actuating pressures, as they are needed to produce the various control pressure differentials for these pressure medium consumers, the line containing the throttling restrictions is blocked by means of an additional pressure regulator whenever a primary pressure medium consumer is in operation and unblocked whenever an auxiliary pressure medium consumer is in operation. A disadvantage here is that the load pressure of the primary pressure medium consumers, which is utilized as actuating pressure for operating said pressure medium consumers is subject to restriction when passing through the throttling restrictions. As a result the actuating pressure takes longer to build up and finally the system dynamics are lower as a result.
A further disadvantage of the prior art hydraulic system is apparent if no implement is mounted on the vehicle, that is to say no auxiliary pressure medium consumer is connected to the hydraulic system of the vehicle. In this case it is possible that due to thermal expansion of the pressure medium inside the load pressure line of the auxiliary pressure medium consumers, which conducts the load pressure, or due to a leakage, pressure medium undesirably flows to the pressure and flow controller of the pump. The effect of this is automatic restriction of the pump even as far as actuation of the assigned pressure relief valve (pump short-circuit).
Hydraulic systems of the type described are predominantly used in high performance agricultural tractors because of the high cost of the load-sensing pump. In less powerful agricultural tractors hydraulic systems with cost-favourable fixed displacement gear pumps as well as load sensing control valves are normally used. A pressure regulator is assigned to such a pump, which in the case of non-operating pressure medium consumers and a control pressure differential of approximately 5 bar returns the conveyed pressure medium to the tank virtually unpressurized. On the other hand if at least one pressure medium consumer is in operation, a system pressure, which is higher by a control pressure differential of approximately 13 bar than its load pressure is built up as a function of the load pressure. Therefore all primary pressure medium consumers mounted on the vehicle can be adequately supplied with pressure medium. As is the case of the prior art hydraulic system however this does not apply to auxiliary pressure medium consumers, in the long supply lines of which substantial pressure losses take place.
It is therefore desirable to provide a hydraulic system of the kind described at the beginning, which, irrespective of whether the hydraulic system is equipped with a variable displacement (load sensing) pump or fixed displacement (gear) pump, can supply sufficient volume and pressure to both the primary pressure medium consumers and the auxiliary consumers without any significant time delay.